1. Field of the Invention
The present invention relates to gas analyzers. In particular, the present invention provides an improved test gas chamber for use with conventional spectrometers as well as automated acousto-optic infrared analyzer systems in which test gases are used for the calibration of such devices.
2. Description of the Prior Art
Conventional on-line in-situ gas spectrometers measure a limited number of gases. These devices are of the NDIR type which use a broad band of light that is centered at the absorption peak of the gas molecule of the species of interest. This broad band is easily selected from all the light frequencies emitted by the source through the use of a passband filter. In order to calibrate such devices, cells which are constructed from light-transmitting materials and which contain a sample of the specific gas of interest are inserted into the path of the light beam in order to calibrate the instrument. When the instrument measures more than one gas, several individual sealed gas cells are selectively inserted into the path of the beam. The up-scale readings given by these known cells can then be extrapolated to a zero value for calibration purposes.
A large and growing market exists for analytical devices which can be used to analyze the reaction products of a wide variety of industrial processes. In addition, on-line real time combustion product analyzers are needed to facilitate the more efficient burning of hydrocarbon fuel. For example, the electric utility industry has been faced with the task of monitoring stack emissions for some years. At first, the monitored emissions consisted of measuring the oxygen concentration and of monitoring smoke opacity in order to control combustion efficiency. Continued increases in the number of regulations have lead to the necessity of monitoring sulfur oxides and nitrogen oxides. Additional constituents will undoubtedly also require monitoring in the future. Proposed regulations in Europe, for example, will require the monitoring of ammonia if it is added to the stack to maintain precipitron efficiency. Recent studies have shown that increased combustion efficiency can be achieved if carbon monoxide is monitored. Therefore, the list of gases which are of interest to commercial activities has rapidly grown to larger and larger numbers. The historical solution has been to installed a separate sensor to monitor each gas. This approach becomes more and more expensive and a less desirable approach as the total number of gases for which monitoring is required increases.
It is known to employ analytical devices which utilize ultraviolet and infrared spectrophotometry, as well as gas and liquid chromatography in order to meet the industrial needs discussed above. Such optical instruments utilize the following important characteristics of the materials being analyzed. A particular molecule has a characteristic absorption spectrum which is dissimilar to that of all other molecules. The spectra of mixtures of molecules are additive and the absorption is proportional to the concentrations of the molecules. Optical absorption spectra can be obtained from any type of sample be it solid, liquid or gas so long as the sample is optically transmissive, and the spectra can be obtained in a non-destructive testing of the sample. Almost all gas molecules absorb infrared radiation. Each molecule has its absorption at specific characteristic wavelengths, so that by monitoring the magnitude of the absorption at the specific wavelengths, the amount of gas present in the stack may be deduced. This is accomplished by transmitting infrared radiation across the stack to a detector. The wavelength of the transmitted radiation is tuned to coincide with the specific wavelength of the gas to be measured. The strength of the detector signal at a given wavelength correlates with the concentration of the absorbing gas at that wavelength. The selective tuning of the infrared radiation is accomplished either by inserting narrow band interference filters in the path, or by inserting cells containing appropriate gas fills in the path. Both of these approaches involve mechanical motion, and the number of cells or filters that can be employed is obviously limited. Thus, reliability and accuracy suffer from the less than ideal properties of these tuning elements.
It has recently been recognized that certain birefringent optical materials which are termed acousto-optic materials can be used as a filter in a spectrum analyzer. U.S. Pat. No. 3,792,287 which is assigned to the assignee of the present invention and incorporated herein by reference teaches the use of an efficient infrared transmissive acousto-optic material, thallium-arsenic-selenide which offers the possibility of operation over the near to mid-infrared spectrum from about 1 micrometer to about 16 micrometers. An automated acousto-optic infrared analyzer system which utilizes acousto-optic technology is described in U.S. Pat. No. 4,490,845 which is assigned to the assignee of the present invention and incorporated herein by reference. This patent teaches an automated acousto-optic tunable filter infrared analyzer system which permits rapid electronic tuning of the filter to a selected infrared bandpass via the acousto-optic interaction with infrared radiation which has passed through a sample. The infrared analyzer system includes means for detecting infrared radiation through the sample to be analyzed, which sample has a predetermined infrared absorption characteristic. Means are provided for directing the infrared radiation through an acousto-optic tunable filter. An acousto-optic tunable filter includes an optically aligned acousto-optic crystal through which infrared radiation is passed at a predetermined angle relative to the crystal optic axis. An acoustic transducer is coupled to the crystal and a variable frequency RF energy source whereby acoustic waves are launched in the crystal in order to interact with the selected narrow bandwidth portion of the polarized infrared radiation to make it distinguishable from the remaining radiation. The tuned or selected narrow bandwidth radiation is a function of the frequency of the RF energy source which is connected to the acoustic transducer of the filter. Infrared radiation detection means are coupled to the filter in order to detect the output filtered infrared radiation and to generate an output signal as a function of the output filtered infrared radiation. Automated computing means are provided with the detection means output electrical signal. The computing means includes means for selectively activating the RF energy source to determine the timing and frequency of RF energy applied to the acoustic transducer to thereby determine the selected or filtered narrow bandwidth infrared wavelength of interest.
A specific application of an acousto-optic infrared analyzer system for monitoring stack gas emissions is disclosed in U.S. patent application Ser. No. 736,199, filed May 20, 1985, now U.S. Pat. No. 4,652,756, and assigned to the assignee of the present invention. In such an acousto-optic tunable filter based spectrometer, a large number of different gases can be measured. In order to establish the correct operating parameters of the system as well as to calibrate the system, test gas requirements could involve a wide range of gases and concentrations. Obviously, it becomes quite costly and very impractical to build a large number of test gas cells either into the device or to provide them for use with the device as is the present practice with on-line gas spectrometers. Additionally, the added cost of the light transmitting windows used to contain test gases for the infrared wavelengths is not desirable.
It is therefore an object of the present invention to provide an improvement to the detector assembly of a spectrometer consisting of a focusing lens and a sealed detector. The addition of a housing designed to mate and seal to the focusing lens and the detector housing or detector window defines a test gas chamber formed between these two light-transmitting elements whereby test gases can be selectively introduced into and removed from this test gas chamber.
It is also an object of this invention to provide a test gas chamber in which the focusing lens serves as the only barrier between the detector means and the process or sample gas being analyzed.
It is a further object of this invention to provide an improvement to an automated acousto-optic infrared analyzer system which improvement consists of an integral test gas chamber.
It remains another object of this invention to provide a test gas chamber design which minimizes the chamber volume and still facilitates the unimpeded focus of the beam onto a detector means.
It is still an object of this invention to provide a test gas chamber with a unique method of test gas introduction whereby a tangential flowpath direction within the gas chamber is established.